context-configured GET requests and serialization config

[micimize]

This is a draft of functionality I added to make PersistedQueriesLink work in graphql_flutter v4:

• Adds a HttpLinkMethod for context-based GET requests
• Adds a RequestSerializationInclusions for configuring serialization

I'm pretty unsure about both APIs, but essentially PersistedQueriesLink wants to set GET in context and have a way to set includeQuery: false.

[brightchip]

Please how do I enable persisted queries ?
There's no PersistedQueriesLink in the graphql_flutter: ^5.0.1-beta.1.
This is a must feature for my project. How do I use it

[micimize]

@brightchip I had a fairly complete (passed all tests) migration done in zino-hofmann/graphql-flutter#756 based on this code, but:

1. I didn't like the RequestSerializationInclusions API here, and thought it needed to be refactored
2. I didn't like the coupling to HttpLink when I think DioLink may be better in the long run.
3. I think PersistedQueriesLink would be better as a standalone link.
4. I stopped contributing to the ecosystem.

If you need this feature, my recommendation would be to:

1. fork my changes here and re-PR them here, maybe figuring out a better way or doing the payload configuration
2. copy the link from https://github.com/zino-app/graphql-flutter/pull/756/files into an independent package that depends on your fork with a git dependency.